Strong-field-induced N₂⁺ Air Lasing In Nitrogen Glow Discharge Plasma

The advantage of the ultrafast femtosecond laser is its high peak power,which causes the excitation and ionization of gaseous atoms and molecules.The molecules/ions excited by the strong field can achieve population inversion,generating the phenomenon of"air lasing".Enhancing the intensity of"air lasing"and explaining the mechanism of air lasers has been studied for a long time.The N₂⁺air lasing at 391 nm induced by strong laser fields in a nitrogen glow discharge plasma is investigated.We find that the 391-nm lasing quenches when the nitrogen gas is electrically discharged.In contrast,the 391-nm fluorescence measured from the side of the filament is strongly enhanced,showing that the discharge promotes the population in the B²?_u⁺(v'=0)state.By comparing the lasing and fluorescence spectra of the nitrogen gas obtained in the discharged and filament-induced plasma,we show that the quenching of N₂⁺lasing comes from the suppression of population inversion between the B²?_u⁺and X²?_g⁺states of N₂⁺,in which a much higher population occurs in the X²?_g⁺state in the discharge plasma.Our results clarify the importance role of the population inversion in generating N₂⁺air lasing,and also provide a possibility for enhancement of the N₂⁺lasing by further manipulation of the population in the X²?_g⁺state in the discharged medium.We found that it has no enhancement effect on N₂⁺air lasing under the condition of glow discharge with 400 nm laser as seed light,but the side fluorescence was increased.At the same time,N₂⁺lasing is suppressed under glow discharge with polarization gating(PG)technology.We found that it is conducive to the generation of B²?_u⁺states of N₂⁺,by observing the side fluorescence.The glow discharge destroys the conversion of the B²?_u⁺and X²?_g⁺states of N₂⁺with producing a large number of X²?_g⁺state of N₂⁺,These experiments provide clues for the subsequent exploration of air laser enhancement,but how to achieve population inversion is an important problem to be solved.